- Email: [email protected]
Thermal properties and crystallization behavior of some TeO2–K2O glasses
Journal of the European Ceramic Society 27 (2007) 1823–1827
Thermal properties and crystallization behavior of some TeO2–K2O glasses b ¨ a,∗ , I. Kabalcı b , M.L. Ovec ¨ ¸ o˘glu a , G. Ozen ¨ B. Oz a
Department of Metallurgical and Materials Engineering, Istanbul Technical University, 34469 Istanbul, Turkey b Department of Physics, Istanbul Technical University, 34469 Istanbul, Turkey Available online 9 June 2006
Abstract The effect of K2 O content on the crystallization of binary TeO2 –K2 O glasses was investigated. Glasses with compositions of (1 − x)TeO2 –xK2 O (where x = 0.05, 0.10, 0.15, in molar ratio) were prepared. DTA curves in the temperature range of 25–700 ◦ C with the heating rate of 10 ◦ C/min were used to determine the thermal properties such as glass transition temperature (Tg ), crystallization peaks (Tp ), and melting temperature (Tm ). Crystallization peaks ranging between 364 and 421 ◦ C were detected on the DTA scans. DTA results revealed that the most stable glasses were formed with the modifier concentrations near the eutectic composition, 0.10 mol. Crystallizing phases and microstructural morphology for each composition were characterized by XRD and SEM. On the basis of XRD measurements, ␣-TeO2 (paratellurite), ␥-TeO2 and K2 Te4 O9 (potassium tetratellurite) phases were detected. The K2 Te4 O9 phase is present in all three glasses, due to the fact that this phase arises because of a phase separation melting of the glass. © 2006 Elsevier Ltd. All rights reserved. Keywords: Microstructure; Thermal properties; Glass
1. Introduction
2. Experimental procedure
Tellurium oxide (TeO2 ) under normal conditions does not have the ability to form glass easily without a modifier like alkali, earth alkali and other glass formers.1 Potassium oxide (K2 O) is a strong modifier for the glass systems and it provides transparency and homogeneity. An understanding of the thermal stability and crystallization behavior is vital to develop and use TeO2 -based glasses.2 Understanding of the crystallization behavior is important for TeO2 -based glasses intended for laser applications during which the glasses will bear thermal loads and thus will be subjected to crystallization.3 The aim of this study is to determine the thermal properties and the crystallization behavior of some (1 − x)TeO2 –xK2 O glasses. For this purpose, three different glass compositions, viz. 0.95 TeO2 –0.05 K2 O, 0.9 TeO2 –0.1 K2 O, 0.85 TeO2 –0.15 K2 O, were investigated using DTA, X-ray diffractometry and SEM techniques.
2.1. Glass synthesis Glass samples were prepared with the compositions of (1 − x)TeO2 –xK2 O where x = 0.05, 0.10 and 0.15 in molar ratio (now hereafter referred to as the 0.95 TeO2 –0.05 K2 O, 0.9 TeO2 –0.1 K2 O, 0.85 TeO2 –0.15 K2 O glasses, respectively). Powders of reagent-grade K2 CO3 (99.995% purity, Chempur Co.) and TeO2 (99.995% purity, Chempur Co.) were used as starting materials. Batches of 7 g in size were thoroughly mixed and melted using a platinum crucible with a closed lid in an electrically heated furnace at 900–950 ◦ C for 30 min, until CO2 evolution ceased. To achieve homogeneity, the cast was crushed, pulverized and reheated at the same temperature for additional 30 min. Following that, the melts were removed from the furnace at 950 ◦ C and quenched by immersing the crucible in a shallow water bath. 2.2. Thermal behavior and microstructural characterization
∗
Corresponding author. ¨ E-mail address: [email protected] (B. Oz).
0955-2219/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2006.04.143
Differential thermal analysis (DTA) scans of as-cast glass specimens were carried out in Thermoflex Rigaku thermal
1824
¨ et al. / Journal of the European Ceramic Society 27 (2007) 1823–1827 B. Oz
analyzer equipped with PTC-10A temperature controller unit to determine the characteristics glass transition temperatures (Tg ), crystallization peaks (Tp ), and melting temperature (Tm ). The glass powder, 50 mg, were heated at the rate of 10 ◦ C/min from room temperature to 700 ◦ C in a platinum crucible and using the same amount of alumina powder as the reference material. Identification of the crystallized phases were determined by X-ray diffraction (XRD) techniques by using a PhilipsTM model PW3710 (Cu K␣ radiation) diffractometer setting in the 2θ range from 10◦ to 90◦ . The crystallized phases were identified by comparing the peak positions and intensities with those in the JCPDS (joint committee on powder diffraction standards) data files. Scanning electron microscopy (SEM) investigations were conducted in JEOLTM JSM 5410 operated at 15 kV and linked with Noran 2100 freedom energy dispersive spectrometer (EDS) attachment. Optical mount specimens were prepared using standard metallographic techniques followed by etching in a 5% HF solution for 20–30 s and coated with carbon. 3. Results and discussion The range of glass formation in the binary tellurite glasses have been documented in the “Tellurite Glasses Handbook”4 for K2 O content between 0.025 and 0.345 mol although some variation is possible depending upon the exact method of sample preparation. After a series of preliminary X-ray diffractometry tests, three compositions of (1 − x)TeO2 –xK2 O glass system with x = 0.05, 0.10 and 0.15 mol were chosen which yielded successful glass formation, depending on the classical quenching method. The glass formation is favored at eutectic compositions5 and TeO2 –K2 O glass system exhibits an eutectic phase at exactly 0.90 TeO2 –0.10 K2 O glass composition. Differential thermal (DTA) investigations were conducted on the as-cast TeO2 –K2 O glasses. Each DTA scan exhibit a small endothermic peak corresponding to the glass transition temperature, Tg , at 310 ◦ C for the 0.95 TeO2 –0.05 K2 O glass, at 295 ◦ C for the 0.9 TeO2 –0.1 K2 O glass and at 275 ◦ C for the 0.85 TeO2 –0.15 K2 O glass (Fig. 1(a)–(c), respectively). The
increase in the K2 O content leads to the decrease of the glass transition temperature values. This result suggests that K2 O acts as a network modifier and TeO2 acts as the network former. Crystallization processes are marked by single exothermic peaks at 394 ◦ C for the 0.95 TeO2 –0.05 K2 O and at 421 ◦ C for the 0.9 TeO2 –0.1 K2 O glass. Two exothermic peaks occurring at 364 and 418 ◦ C were observed for the 0.85 TeO2 –0.15 K2 O glass (Fig. 1(c)) and it is evident that both peaks can be attributed to the formation and/or transformation of crystalline phases. Glass forming tendency, Kg , given by Eq. (1), can be determine in order to compare devitrification tendency of the glasses. Low values of Kg suggests high tendency of crystallization:6 Kg =
Tc − Tg Tm − T c
(1)
Glass forming tendency values are calculated by using the temperatures, Tg , Tp , Tm (Table 1), obtained from DTA curves of the 0.95 TeO2 –0.05 K2 O, 0.9 TeO2 –0.1 K2 O and 0.85 TeO2 –0.15 K2 O glasses scanned at a rate of 10 ◦ C/min. As seen in Table 1, 0.9 TeO2 –0.1 K2 O is the most susceptible to form glass among three compositions, with a Kg value of 2.06. On the basis of DTA results (Fig. 1), XRD scans were carried out to identify the crystallizing phases in the glassy matrix above the peak temperatures for all compositions. Fig. 2(a)–(c) are the XRD patterns taken from the heat-treated and the as-cast samples of the 0.95 TeO2 –0.05 K2 O, 0.9 TeO2 –0.1 K2 O and 0.85 TeO2 –0.15 K2 O glasses, respectively. The heat-treated glass samples were prepared by heating the as-cast glasses 10–15 ◦ C above the crystallization peak temperature for 30 min, followed by quenching in air. Hart7 reported that the formation of the K2 Te4 O9 phase, as a result of a phase separation in TeO2 –K2 O glasses with varying between 5 and 30 mol% K2 O content. This phase which has the same symmetry as the crystal, P21 /c, which is a centrosymmetric monoclinic space group but the lattice is tetragonal, with lattice parameters of 0.757, 0.773 and 1.782 nm, This is the reason why the K2 Te4 O9 phase is detected in the XRD scans (Fig. 2) of heat-treated and as-cast samples of TeO2 –K2 O glasses in the present investigation. In other words, the diffraction angle values (2θ) of 12.5◦ , 15◦ and 20◦ detected in all figures of Fig. 2 are the characteristic angles for the K2 Te4 O9 phase.8 The XRD pattern of the 0.95 TeO2 –0.05 K2 O glass crystallized at 425 ◦ C (Fig. 2(a)) reveals the presence of the ␥-TeO2 and ␣-TeO2 phases in addition to the K2 Te4 O9 phase. Whereas the stable ␣-TeO2 phase (paratellurite) has a tetragonal crystal structure with lattice parameters a = 0.481 nm and c = 0.761 nm,9 Table 1 Values of glass forming tendency, Kg , and glass transition, Tg , crystallization, Tp and melting, Tm , temperatures of the (1 − x)TeO2 –xK2 O glasses
Fig. 1. DTA curves of as-cast samples of 0.95 TeO2 –0.05 K2 O glass (a), 0.9 TeO2 –0.1 K2 O glass (b) and 0.85 TeO2 –0.15 K2 O glass (c) scanned at a rate of 10 ◦ C/min.
K2 O content (mol%)
Tg (◦ C)
Tp1 (◦ C)
Tp2 (◦ C)
Tm (◦ C)
Kg =
5 10 15
310 297 275
394 421 364
– – 418
472 481 485
1.07 2.06 0.73
(Tc −Tg ) (Tm −Tc )
¨ et al. / Journal of the European Ceramic Society 27 (2007) 1823–1827 B. Oz
1825
Fig. 2. XRD scans taken from: (a) the 0.95 TeO2 –0.05 K2 O glass in the as-cast condition and after heat-treating at 425 ◦ C; (b) the 0.9 TeO2 –0.1 K2 O glass in the as-cast condition and after heat-treating at 440 ◦ C; (c) the 0.85 TeO2 –0.15 K2 O glass in the as-cast condition and after heat-treating at 385 and 430 ◦ C.
metastable phase has an orthorhombic crystal structure with lattice parameters a = 0.845 nm, b = 0.499 nm and c = 0.430 nm.10 On the basis of XRD pattern of 0.9 TeO2 –0.1 K2 O sample heat-treated at 440 ◦ C (Fig. 2(b)) comprise the ␣-TeO2 , ␥-TeO2 and K2 Te4 O9 phases similar to the 0.95 TeO2 –0.05 K2 O sample. Fig. 2(c) shows the XRD pattern of the 0.85 TeO2 –0.15 K2 O glass in the as-cast condition and crystallized at 385 and 430 ◦ C. All three crystallizing phases (␣-TeO2 , ␥-TeO2 and K2 Te4 O9 ) co-exist at 385 ◦ C (above the first exotherm in Fig. 1), as expected. On the other hand, the amount of ␥-TeO2 decreased the 430 ◦ C inferring that the second exotherm in Fig. 1 is a result of ␥-TeO2 → ␣-TeO2 transformation. Existence of K2 Te4 O9 and TeO2 phases were also verified by SEM and EDS analyses. SEM investigations were also performed for the heat-treated samples to identify the morphology of the resultant microstructures after crystallization. All SEM micrographs are taken in the secondary electron imaging (SEI) mode.
Fig. 3(a) is a representative SEM micrograph of the 0.95 TeO2 –0.05 K2 O glass samples heated to 425 ◦ C followed by air-quenching. EDS analyses taken from the elongated crystalline regions (85.2 ± 0.5 wt.% Te, 0.08 ± 0.4 wt.% K and 13 ± 0.4 wt.% O) confirms that the crystalline regions are composed of a TeO2 phase. Fig. 3(b) is a SEM micrograph of the 0.9 TeO2 –0.1 K2 O sample heat-treated at 440 ◦ C showing square-shaped crystals varying between 7 and 10 m width, 7 and 10 m in length. EDS analyses taken from three different locations (regions B in Fig. 3(b)) show that these crystals contained 74.2 ± 0.3 wt.% Te, 6.5 ± 0.4 wt.% K and 18.2 ± 0.3 wt.% O, indicating that the crystals consist of the TeO2 and K2 Te4 O9 phases. Fig. 3(c) is a SEM micrograph of 0.85 TeO2 –0.15 K2 O glass, heat-treated at 385 ◦ C (first exotherm in Fig. 1). EDS spectra taken from the crystalline structures (regions C in Fig. 3(c)) shows that the amorphous glass matrix in the bulk of the
1826
¨ et al. / Journal of the European Ceramic Society 27 (2007) 1823–1827 B. Oz
Fig. 3. Typical SEM micrographs taken from the crystalline regions of (a) the 0.95 TeO2 –0.05 K2 O sample heat-treated at 425 ◦ C; (b) 0.9 TeO2 –0.1 K2 O sample heat-treated at 440 ◦ C; (c) the 0.85 TeO2 –0.15 K2 O sample heat-treated at 385 ◦ C and (d) the same glass heat-treated at 430 ◦ C.
sample contained 85.2 ± 0.3 wt.% Te, 5.5 ± 0.3 wt.% K, 9.8 ± 0.5 wt.% O. Fig. 3(d) is a SEM micrograph from different locations of the 0.85 TeO2 –0.15 K2 O sample crystallized at 430 ◦ C, showing rectangular-shaped crystals stacked in 2D-orientation having crystal sizes ranging between 5 and 10 m in width, 5 and 15 m in length. EDS spectra taken from the crystalline regions (regions D in Fig. 3(d)) shows that the regions had the chemical stoichiometry of 67.7 ± 0.5 wt.% Te, 11.2 ± 0.5 wt.% K and 21.3 ± 0.4 wt.% O, indicating that these are K2 Te4 O9 -rich crystals surrounded by a glassy matrix, regions G. 4. Conclusions In this study, thermal properties of (1 − x)TeO2 –xK2 O binary glasses were investigated in order to examine the effect of K2 O content. According to glass forming tendency values the glass having a K2 O concentration of roughly 0.10 mol is the easiest to form glass. The peak crystallization temperatures vary from 364 to 421 ◦ C with the increase in the heating rate. Two exotherms were observed in the DTA curves of 0.85 TeO2 –0.15 K2 O glass while the 0.95 TeO2 –0.05 K2 O and 0.9 TeO2 –0.1 K2 O glasses having one for each.
XRD investigations shows that, all the heat-treated glasses contains K2 Te4 O9 , ␣-TeO2 and ␥-TeO2 phases at varying amounts according to the composition and increase in the amount of K2 O content inhibits the crystallization of the TeO2 phase. SEM studies reveals; the second exotherm is associated to the crystallization of the K2 Te4 O9 phase for the 0.85 TeO2 –0.15 K2 O glass, while the TeO2 phase is dominant for the 0.95 TeO2 –0.05 K2 O glass. Acknowledgements The authors express their grateful thanks to Prof. Dr. M. ¨ Urgen for the use of his experimental equipment. The authors also thank to Research Assistant Murat Telli for recording the XRD patterns and to Sevgin T¨urkeli for her support in SEM study. This investigation was supported by the State Planning Organization (DPT) under the project title “Advanced Technologies in Engineering”. References 1. Sidkey, M. A. and Gaafar, M. S., Ultrasonic studies on network structures of ternary TeO2 –WO3 –K2 O glass system. Physica B, 2004, 348, 46–55.
¨ et al. / Journal of the European Ceramic Society 27 (2007) 1823–1827 B. Oz 2. El-Mallawany, R., J. Appl. Phys., 1992, 72, 1774. ¨ ¸ o˘glu, M. L., Ozalp, M. R., Ozen, ¨ 3. Ovec G., Altın, F. and Kalem, V., Crystallization behavior of some TeO2 –ZnO glasses. Key Eng. Mater., 2004, 264, 1891–1894. 4. El-Mallawany, R. A. H., Tellurite Glasses Handbook: Physical Properties and Data. CRC Press, Boca Raton, Florida, 2002, pp. 25– 27. 5. Varshneya, A. K., Fundamentals of Inorganic Glasses. Academic Press, San Diego, 1994. 6. El-Mallawany, R., Devitrification and vitrification of tellurite glasses. J. Mater. Sci. Electron., 1995, 6, 1–3.
1827
7. Hart, R. T., Structures and optical properties of tellurite glass and glass ceramics. PhD thesis. Department of Chemistry, Indiana University, Indiana, 2004. 8. Powder Diffraction Files: Card No. 28–806, Database Edition, Joint Committee on Powder Diffraction Standards (JCPDS), Stanworth, PA, USA, 1992. 9. Powder Diffraction Files: Card No. 42–1365, Database Edition, Joint Committee on Powder Diffraction Standards (JCPDS), Stanworth, PA, USA, 1992. 10. Blanchandin, S., Marchet, P. and Thomas, P., New investigations within the TeO2 –WO3 system. J. Mater. Sci., 1999, 34, 4285–4292.
Thermal properties and crystallization behavior of some TeO2–K2O glasses b ¨ a,∗ , I. Kabalcı b , M.L. Ovec ¨ ¸ o˘glu a , G. Ozen ¨ B. Oz a
Department of Metallurgical and Materials Engineering, Istanbul Technical University, 34469 Istanbul, Turkey b Department of Physics, Istanbul Technical University, 34469 Istanbul, Turkey Available online 9 June 2006
Abstract The effect of K2 O content on the crystallization of binary TeO2 –K2 O glasses was investigated. Glasses with compositions of (1 − x)TeO2 –xK2 O (where x = 0.05, 0.10, 0.15, in molar ratio) were prepared. DTA curves in the temperature range of 25–700 ◦ C with the heating rate of 10 ◦ C/min were used to determine the thermal properties such as glass transition temperature (Tg ), crystallization peaks (Tp ), and melting temperature (Tm ). Crystallization peaks ranging between 364 and 421 ◦ C were detected on the DTA scans. DTA results revealed that the most stable glasses were formed with the modifier concentrations near the eutectic composition, 0.10 mol. Crystallizing phases and microstructural morphology for each composition were characterized by XRD and SEM. On the basis of XRD measurements, ␣-TeO2 (paratellurite), ␥-TeO2 and K2 Te4 O9 (potassium tetratellurite) phases were detected. The K2 Te4 O9 phase is present in all three glasses, due to the fact that this phase arises because of a phase separation melting of the glass. © 2006 Elsevier Ltd. All rights reserved. Keywords: Microstructure; Thermal properties; Glass
1. Introduction
2. Experimental procedure
Tellurium oxide (TeO2 ) under normal conditions does not have the ability to form glass easily without a modifier like alkali, earth alkali and other glass formers.1 Potassium oxide (K2 O) is a strong modifier for the glass systems and it provides transparency and homogeneity. An understanding of the thermal stability and crystallization behavior is vital to develop and use TeO2 -based glasses.2 Understanding of the crystallization behavior is important for TeO2 -based glasses intended for laser applications during which the glasses will bear thermal loads and thus will be subjected to crystallization.3 The aim of this study is to determine the thermal properties and the crystallization behavior of some (1 − x)TeO2 –xK2 O glasses. For this purpose, three different glass compositions, viz. 0.95 TeO2 –0.05 K2 O, 0.9 TeO2 –0.1 K2 O, 0.85 TeO2 –0.15 K2 O, were investigated using DTA, X-ray diffractometry and SEM techniques.
2.1. Glass synthesis Glass samples were prepared with the compositions of (1 − x)TeO2 –xK2 O where x = 0.05, 0.10 and 0.15 in molar ratio (now hereafter referred to as the 0.95 TeO2 –0.05 K2 O, 0.9 TeO2 –0.1 K2 O, 0.85 TeO2 –0.15 K2 O glasses, respectively). Powders of reagent-grade K2 CO3 (99.995% purity, Chempur Co.) and TeO2 (99.995% purity, Chempur Co.) were used as starting materials. Batches of 7 g in size were thoroughly mixed and melted using a platinum crucible with a closed lid in an electrically heated furnace at 900–950 ◦ C for 30 min, until CO2 evolution ceased. To achieve homogeneity, the cast was crushed, pulverized and reheated at the same temperature for additional 30 min. Following that, the melts were removed from the furnace at 950 ◦ C and quenched by immersing the crucible in a shallow water bath. 2.2. Thermal behavior and microstructural characterization
∗
Corresponding author. ¨ E-mail address: [email protected] (B. Oz).
0955-2219/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2006.04.143
Differential thermal analysis (DTA) scans of as-cast glass specimens were carried out in Thermoflex Rigaku thermal
1824
¨ et al. / Journal of the European Ceramic Society 27 (2007) 1823–1827 B. Oz
analyzer equipped with PTC-10A temperature controller unit to determine the characteristics glass transition temperatures (Tg ), crystallization peaks (Tp ), and melting temperature (Tm ). The glass powder, 50 mg, were heated at the rate of 10 ◦ C/min from room temperature to 700 ◦ C in a platinum crucible and using the same amount of alumina powder as the reference material. Identification of the crystallized phases were determined by X-ray diffraction (XRD) techniques by using a PhilipsTM model PW3710 (Cu K␣ radiation) diffractometer setting in the 2θ range from 10◦ to 90◦ . The crystallized phases were identified by comparing the peak positions and intensities with those in the JCPDS (joint committee on powder diffraction standards) data files. Scanning electron microscopy (SEM) investigations were conducted in JEOLTM JSM 5410 operated at 15 kV and linked with Noran 2100 freedom energy dispersive spectrometer (EDS) attachment. Optical mount specimens were prepared using standard metallographic techniques followed by etching in a 5% HF solution for 20–30 s and coated with carbon. 3. Results and discussion The range of glass formation in the binary tellurite glasses have been documented in the “Tellurite Glasses Handbook”4 for K2 O content between 0.025 and 0.345 mol although some variation is possible depending upon the exact method of sample preparation. After a series of preliminary X-ray diffractometry tests, three compositions of (1 − x)TeO2 –xK2 O glass system with x = 0.05, 0.10 and 0.15 mol were chosen which yielded successful glass formation, depending on the classical quenching method. The glass formation is favored at eutectic compositions5 and TeO2 –K2 O glass system exhibits an eutectic phase at exactly 0.90 TeO2 –0.10 K2 O glass composition. Differential thermal (DTA) investigations were conducted on the as-cast TeO2 –K2 O glasses. Each DTA scan exhibit a small endothermic peak corresponding to the glass transition temperature, Tg , at 310 ◦ C for the 0.95 TeO2 –0.05 K2 O glass, at 295 ◦ C for the 0.9 TeO2 –0.1 K2 O glass and at 275 ◦ C for the 0.85 TeO2 –0.15 K2 O glass (Fig. 1(a)–(c), respectively). The
increase in the K2 O content leads to the decrease of the glass transition temperature values. This result suggests that K2 O acts as a network modifier and TeO2 acts as the network former. Crystallization processes are marked by single exothermic peaks at 394 ◦ C for the 0.95 TeO2 –0.05 K2 O and at 421 ◦ C for the 0.9 TeO2 –0.1 K2 O glass. Two exothermic peaks occurring at 364 and 418 ◦ C were observed for the 0.85 TeO2 –0.15 K2 O glass (Fig. 1(c)) and it is evident that both peaks can be attributed to the formation and/or transformation of crystalline phases. Glass forming tendency, Kg , given by Eq. (1), can be determine in order to compare devitrification tendency of the glasses. Low values of Kg suggests high tendency of crystallization:6 Kg =
Tc − Tg Tm − T c
(1)
Glass forming tendency values are calculated by using the temperatures, Tg , Tp , Tm (Table 1), obtained from DTA curves of the 0.95 TeO2 –0.05 K2 O, 0.9 TeO2 –0.1 K2 O and 0.85 TeO2 –0.15 K2 O glasses scanned at a rate of 10 ◦ C/min. As seen in Table 1, 0.9 TeO2 –0.1 K2 O is the most susceptible to form glass among three compositions, with a Kg value of 2.06. On the basis of DTA results (Fig. 1), XRD scans were carried out to identify the crystallizing phases in the glassy matrix above the peak temperatures for all compositions. Fig. 2(a)–(c) are the XRD patterns taken from the heat-treated and the as-cast samples of the 0.95 TeO2 –0.05 K2 O, 0.9 TeO2 –0.1 K2 O and 0.85 TeO2 –0.15 K2 O glasses, respectively. The heat-treated glass samples were prepared by heating the as-cast glasses 10–15 ◦ C above the crystallization peak temperature for 30 min, followed by quenching in air. Hart7 reported that the formation of the K2 Te4 O9 phase, as a result of a phase separation in TeO2 –K2 O glasses with varying between 5 and 30 mol% K2 O content. This phase which has the same symmetry as the crystal, P21 /c, which is a centrosymmetric monoclinic space group but the lattice is tetragonal, with lattice parameters of 0.757, 0.773 and 1.782 nm, This is the reason why the K2 Te4 O9 phase is detected in the XRD scans (Fig. 2) of heat-treated and as-cast samples of TeO2 –K2 O glasses in the present investigation. In other words, the diffraction angle values (2θ) of 12.5◦ , 15◦ and 20◦ detected in all figures of Fig. 2 are the characteristic angles for the K2 Te4 O9 phase.8 The XRD pattern of the 0.95 TeO2 –0.05 K2 O glass crystallized at 425 ◦ C (Fig. 2(a)) reveals the presence of the ␥-TeO2 and ␣-TeO2 phases in addition to the K2 Te4 O9 phase. Whereas the stable ␣-TeO2 phase (paratellurite) has a tetragonal crystal structure with lattice parameters a = 0.481 nm and c = 0.761 nm,9 Table 1 Values of glass forming tendency, Kg , and glass transition, Tg , crystallization, Tp and melting, Tm , temperatures of the (1 − x)TeO2 –xK2 O glasses
Fig. 1. DTA curves of as-cast samples of 0.95 TeO2 –0.05 K2 O glass (a), 0.9 TeO2 –0.1 K2 O glass (b) and 0.85 TeO2 –0.15 K2 O glass (c) scanned at a rate of 10 ◦ C/min.
K2 O content (mol%)
Tg (◦ C)
Tp1 (◦ C)
Tp2 (◦ C)
Tm (◦ C)
Kg =
5 10 15
310 297 275
394 421 364
– – 418
472 481 485
1.07 2.06 0.73
(Tc −Tg ) (Tm −Tc )
¨ et al. / Journal of the European Ceramic Society 27 (2007) 1823–1827 B. Oz
1825
Fig. 2. XRD scans taken from: (a) the 0.95 TeO2 –0.05 K2 O glass in the as-cast condition and after heat-treating at 425 ◦ C; (b) the 0.9 TeO2 –0.1 K2 O glass in the as-cast condition and after heat-treating at 440 ◦ C; (c) the 0.85 TeO2 –0.15 K2 O glass in the as-cast condition and after heat-treating at 385 and 430 ◦ C.
metastable phase has an orthorhombic crystal structure with lattice parameters a = 0.845 nm, b = 0.499 nm and c = 0.430 nm.10 On the basis of XRD pattern of 0.9 TeO2 –0.1 K2 O sample heat-treated at 440 ◦ C (Fig. 2(b)) comprise the ␣-TeO2 , ␥-TeO2 and K2 Te4 O9 phases similar to the 0.95 TeO2 –0.05 K2 O sample. Fig. 2(c) shows the XRD pattern of the 0.85 TeO2 –0.15 K2 O glass in the as-cast condition and crystallized at 385 and 430 ◦ C. All three crystallizing phases (␣-TeO2 , ␥-TeO2 and K2 Te4 O9 ) co-exist at 385 ◦ C (above the first exotherm in Fig. 1), as expected. On the other hand, the amount of ␥-TeO2 decreased the 430 ◦ C inferring that the second exotherm in Fig. 1 is a result of ␥-TeO2 → ␣-TeO2 transformation. Existence of K2 Te4 O9 and TeO2 phases were also verified by SEM and EDS analyses. SEM investigations were also performed for the heat-treated samples to identify the morphology of the resultant microstructures after crystallization. All SEM micrographs are taken in the secondary electron imaging (SEI) mode.
Fig. 3(a) is a representative SEM micrograph of the 0.95 TeO2 –0.05 K2 O glass samples heated to 425 ◦ C followed by air-quenching. EDS analyses taken from the elongated crystalline regions (85.2 ± 0.5 wt.% Te, 0.08 ± 0.4 wt.% K and 13 ± 0.4 wt.% O) confirms that the crystalline regions are composed of a TeO2 phase. Fig. 3(b) is a SEM micrograph of the 0.9 TeO2 –0.1 K2 O sample heat-treated at 440 ◦ C showing square-shaped crystals varying between 7 and 10 m width, 7 and 10 m in length. EDS analyses taken from three different locations (regions B in Fig. 3(b)) show that these crystals contained 74.2 ± 0.3 wt.% Te, 6.5 ± 0.4 wt.% K and 18.2 ± 0.3 wt.% O, indicating that the crystals consist of the TeO2 and K2 Te4 O9 phases. Fig. 3(c) is a SEM micrograph of 0.85 TeO2 –0.15 K2 O glass, heat-treated at 385 ◦ C (first exotherm in Fig. 1). EDS spectra taken from the crystalline structures (regions C in Fig. 3(c)) shows that the amorphous glass matrix in the bulk of the
1826
¨ et al. / Journal of the European Ceramic Society 27 (2007) 1823–1827 B. Oz
Fig. 3. Typical SEM micrographs taken from the crystalline regions of (a) the 0.95 TeO2 –0.05 K2 O sample heat-treated at 425 ◦ C; (b) 0.9 TeO2 –0.1 K2 O sample heat-treated at 440 ◦ C; (c) the 0.85 TeO2 –0.15 K2 O sample heat-treated at 385 ◦ C and (d) the same glass heat-treated at 430 ◦ C.
sample contained 85.2 ± 0.3 wt.% Te, 5.5 ± 0.3 wt.% K, 9.8 ± 0.5 wt.% O. Fig. 3(d) is a SEM micrograph from different locations of the 0.85 TeO2 –0.15 K2 O sample crystallized at 430 ◦ C, showing rectangular-shaped crystals stacked in 2D-orientation having crystal sizes ranging between 5 and 10 m in width, 5 and 15 m in length. EDS spectra taken from the crystalline regions (regions D in Fig. 3(d)) shows that the regions had the chemical stoichiometry of 67.7 ± 0.5 wt.% Te, 11.2 ± 0.5 wt.% K and 21.3 ± 0.4 wt.% O, indicating that these are K2 Te4 O9 -rich crystals surrounded by a glassy matrix, regions G. 4. Conclusions In this study, thermal properties of (1 − x)TeO2 –xK2 O binary glasses were investigated in order to examine the effect of K2 O content. According to glass forming tendency values the glass having a K2 O concentration of roughly 0.10 mol is the easiest to form glass. The peak crystallization temperatures vary from 364 to 421 ◦ C with the increase in the heating rate. Two exotherms were observed in the DTA curves of 0.85 TeO2 –0.15 K2 O glass while the 0.95 TeO2 –0.05 K2 O and 0.9 TeO2 –0.1 K2 O glasses having one for each.
XRD investigations shows that, all the heat-treated glasses contains K2 Te4 O9 , ␣-TeO2 and ␥-TeO2 phases at varying amounts according to the composition and increase in the amount of K2 O content inhibits the crystallization of the TeO2 phase. SEM studies reveals; the second exotherm is associated to the crystallization of the K2 Te4 O9 phase for the 0.85 TeO2 –0.15 K2 O glass, while the TeO2 phase is dominant for the 0.95 TeO2 –0.05 K2 O glass. Acknowledgements The authors express their grateful thanks to Prof. Dr. M. ¨ Urgen for the use of his experimental equipment. The authors also thank to Research Assistant Murat Telli for recording the XRD patterns and to Sevgin T¨urkeli for her support in SEM study. This investigation was supported by the State Planning Organization (DPT) under the project title “Advanced Technologies in Engineering”. References 1. Sidkey, M. A. and Gaafar, M. S., Ultrasonic studies on network structures of ternary TeO2 –WO3 –K2 O glass system. Physica B, 2004, 348, 46–55.
¨ et al. / Journal of the European Ceramic Society 27 (2007) 1823–1827 B. Oz 2. El-Mallawany, R., J. Appl. Phys., 1992, 72, 1774. ¨ ¸ o˘glu, M. L., Ozalp, M. R., Ozen, ¨ 3. Ovec G., Altın, F. and Kalem, V., Crystallization behavior of some TeO2 –ZnO glasses. Key Eng. Mater., 2004, 264, 1891–1894. 4. El-Mallawany, R. A. H., Tellurite Glasses Handbook: Physical Properties and Data. CRC Press, Boca Raton, Florida, 2002, pp. 25– 27. 5. Varshneya, A. K., Fundamentals of Inorganic Glasses. Academic Press, San Diego, 1994. 6. El-Mallawany, R., Devitrification and vitrification of tellurite glasses. J. Mater. Sci. Electron., 1995, 6, 1–3.
1827
7. Hart, R. T., Structures and optical properties of tellurite glass and glass ceramics. PhD thesis. Department of Chemistry, Indiana University, Indiana, 2004. 8. Powder Diffraction Files: Card No. 28–806, Database Edition, Joint Committee on Powder Diffraction Standards (JCPDS), Stanworth, PA, USA, 1992. 9. Powder Diffraction Files: Card No. 42–1365, Database Edition, Joint Committee on Powder Diffraction Standards (JCPDS), Stanworth, PA, USA, 1992. 10. Blanchandin, S., Marchet, P. and Thomas, P., New investigations within the TeO2 –WO3 system. J. Mater. Sci., 1999, 34, 4285–4292.